Papers by Author: Yang Liu

Abstract: The effects of strain rate on the tensile deformation behavior of quenching and partitioning (Q&P) steel applied to the auto industry were investigated. The results indicated that the strength of Q&P steel raised with increasing strain rate. The variation of elongation which presented the trend of declining (10^-4 s^-1~10¹ s^-1) followed by rising to the peak (8×10¹ s^-1) then falling again (10² s^-1~10³ s^-1), is mainly caused by the transformation from retained austenite to martensite, namely the TRIP effect.

Abstract: The pulse current was performed on the IN718 alloy during tensile test at room temperature and elevated temperature in the present study. The effect of pulse current on the deformation behavior of the alloy and the mechanisms were investigated. The results show that the deformation resistance decreases and the elongation increases significantly of IN718 alloy during tensile test at room temperature and 1073 K with the increase of pulse current. And the effect of pulse current on the strength and plasticity of the alloy increases as pulse current energy increases. The electroplastic effect of pulse current can promote the dislocation motion, and thus decrease the deformation resistance of the alloy during the tensile deformation. The pulse current reduces the starting temperature of recrystallization, promotes the recrystalllization nucleation, and thus makes it possible that the dynamic recrystallization would occur at lower temperature than that in the conventional temperature field. This is the main reason for the decrease of deformation resistance and the increase of plastic deformation ability of IN718 alloy under pulse current.

Abstract: The microstructures and mechanical properties of the Ni-22Cr-14W-2Mo superalloys with various La contents have been investigated. The results indicate that the grain diameters of the alloys with different La additions are similar to that of the alloy without La. It has also been found that the Vickers Hardness of the alloys increases with the increasing of the La content form 0.000% to 0.087%. The tensile properties of the alloys can be improved by La additions, whereas the σ_0.2 proof strength and the elongations of the alloys first increase and then decrease with the increasing of the La content form 0.000% to 0.087%. The alloy with 0.026% La shows the best tensile properties. This result ascribes to the different types of the La-riched phases and the different sizes of them. The La-Ni intermetallic phases with large size precipitating in the alloys have deleterious effects on the improvement of the mechanical properties, if the La content is too high.

Abstract: To reduce fuel consumption and greenhouse gas emission, dual phase (DP) steels have been considered for automotive applications due to their higher tensile strength, better initial work hardening along with larger elongation compared to conventional grade of steels. In such applications, which would create potential safety and reliability issues under dynamic loading, the mechanical behavior of DP steel considering the strain rate must be examined. In the present study, the dynamic tensile behavior of DP600 steel sheets was investigated using a high-speed tensile testing machine at various strain rates. And the quasi-static tensile testing was also conducted on the steel to understand the effect of the strain rate on the tensile property. The fracture mechanisms of the steel were also analyzed. The results show that the mechanical properties of DP600 steel are noticeably influenced by the strain rates. As the strain rate increases, the strength of the steel increases and the obvious yield phenomenon can be observed when the strain rate is above 0.01 s^-1. The fracture elongation of DP600 steels decreases with increasing strain rate from 0.001 to 1 s^-1, then increases up to the strain rate of 100 s^-1 and reaches the lowest value at the strain rate of 1000 s^-1. DP600 steel sheet exhibit typical ductile fracture characteristics with dimples morphology of the facture surface when tensile deformed at various strain rates.

Abstract: We investigated the structural evolution of Polystyrene films at various conditions through multi-wavelength ellipsometry. We studied the thickness of film dependent of the process of swelling/shrinking, and the influence of the intensity of atmosphere，combining with PM (polarization microscope) to characterize the surface morphology. Results indicated that the increase and reduction of the film thickness are non-linear. Maximum swelling ratio and the variation of film thickness depend on the initial thickness of film and the intensity of atmosphere. Furthermore, the results demonstrated that in-situ multi-wavelength ellipsometry is an unique means to investigate the behavior of swelling and shrinking of film.

Abstract: Effect of electric field treatment with different intensities on vacancy of Inconel 718 aged at 1073 K was investigated. The positron annihilation lifetime (PAL) and corresponding concentration of vacancies were measured by positron annihilation technique. It was found that atomic vibration at lattice site can be promoted by an external electric field performed on Inconel 718 superalloy during aged at elevated temperature. Some atoms owing the higher vacancy formation energy or larger radius will jump out from the normal lattice sites and form defects, which is leading to the increase of PAL of monovacancy. Under continuous effect on electric field, monovacancies will transform to three-dimensional vacancies of lower energy and vacancy clusters. Moreover, the vacancy concentration of Inconel 718 superalloy can be improved evidently by electric field treatment, and the average PAL of vacancies can be up to 6.3% when the electric field intensity is up to 8 kV/cm, which is very useful for controlling the microstructures evolution and properties of the superalloy, such as solute redistribution and precipitations.

Abstract: The isothermal oxidation behaviour under static atmosphere of a new directionally solidified Ni-base superalloy was investigated. The results showed that the oxidation kinetics curves of the alloy follow parabolic law in the temperature range of 750-950°C. The diffusion activation energy Q is to be about 244.86 kJ•mol-1 and the oxidation within this temperature range is mainly controlled by Cr3+ diffusion among Cr2O3 oxidation film. The oxidation resistance grade of the alloy is perfect anti-oxidation within 750-850°C, and anti-oxidation within 900-950°C. The oxidation film can be divided into TiO2, Cr2O3+TiTaO4+NiCr2O4, Al2O3 and TiN layers, from the surface to inside. Priority oxidation and the inside oxidation of Al are considered with the oxidation of Ti and the depletion of Cr, and it is the key point for the improving of oxidation resistance.

Abstract: The effects of electropulsing treatment on the precipitation behaviour of grain boundary carbides in GH3044 alloy were investigated. The results showed that the initial temperature of precipitation of M23C6 type carbides on grain boundary could be decreased by electropulsing treatment under the condition of high current density of 10.0kA/mm2 with a frequency of 5Hz and pulse width of 15s. The volume percentage of M23C6 type carbide was greatly increased to 274.60% comparing with that of the aging treatment at the same temperature. However, the precipitation of M23C6 type carbide was inhibited by electropulsing treatment under the condition of high frequency of 45Hz with the current density of 2kA/mm2 and pulse width of 15s. The volume percentage of M23C6 type carbide was decreased to 18.81% comparing with that of the aging treatment at the same temperature. It has been found that the diffusion of solute atom in the alloy can be promoted by the electric effect with the electropulsing. As a result, the thermodynamic condition and kinetics of the precipitation of M23C6 type carbide were changed, and both the initial and peak temperatures of precipitation were decreased.

Abstract: Effects of thermal exposure on low cycle fatigue behavior of Ti600 alloy were investigated by LSCM, SEM and TEM. The results demonstrated that both the NTE specimens and the TE specimens showed the cyclic softening, within a total strain amplitude range from ±0.45% to ±1.00%. Since the α2 phase precipitated in the αp phase during thermal exposure, the resistance of fatigue crack propagation of αp phase could be increased by the precipitation of α2 phase. Therefore, the low cycle fatigue (LCF) lives of Ti600 alloy after thermal exposure were longer than those without thermal exposure, at the same total strain amplitude.

Abstract: Compression test for GH4586 superalloy was carried out at 1273 K and 1373 K with stain from 20% to 60%. The characteristics of dynamic recrystallization were investigated by confocal laser scanning microscope, TEM and EBSD. It was found that at 1273 K dynamic recrystallization mainly took place along preexisting grain boundaries creating a necklace structure. It was confirmed by TEM observations that the first layer of the necklace structure was formed by the mechanism of bulging of preexisting grain boundaries and the following layers were nucleated at the triple junctions of former dynamic recrystallization grains. When specimens were deformed with different strains at 1373 K, dynamic recrystallization was almost completed. Our observation showed that twinning played an important role during dynamic recrystallization by promoting dynamic recrystallization grains growing. Substructures of grains were constructed by the reconfiguration of dislocations hindered at large ' particles in interior of initial grains during dynamic recovery.